The present invention generally relates to a motor vehicle and more particularly, to an antiskid device for utilizing a braking force efficiently through detection of a locking state of wheels of the motor vehicle.
Generally, an antiskid device is arranged as shown in FIG. 1. Initially, an AC voltage supplied from a wheel speed sensor R is converted into pulses by an interface circuit 22. The pulses are counted by a pulse processing circuit 23 so as to be input, as a wheel speed signal, to a circuit 24 for performing arithmetic operation and detecting a locking state of wheels of a motor vehicle. In this circuit 24, arithmetic operation of an estimated vehicle speed, a deceleration or an acceleration is performed on the basis of the wheel speed signal. When the circuit 24 detects that the wheels are in a state towards locking through comparison between the arithmetic results and various threshold values, the circuit 24 gives a command of reducing a braking pressure. Meanwhile, when the wheels are in a state towards recovery from locking, the circuit 24 issues a command of increasing the braking pressure. Furthermore, under a certain condition, the circuit 24 gives a command of maintaining the braking pressure even during issuance of the command of increasing or decreasing the braking pressure.
In the case where the command of decreasing the braking pressure has been given from the circuit 24, a solenoid driving circuit 25 energizes solenoids SOL1 and SOL2. Thus, pressure control valves V1 and V2 are displaced in the rightward direction in FIG. 1. Therefore, the pressure control valve V1 shuts off a brake fluid pressure circuit from a brake fluid pressure generator composed of a master cylinder 26 and an accumulator 29 and the pressure control valve V2 establishes communication of a brake fluid pressure circuit from a wheel cylinder 27 to a reservoir 28, while brake fluid discharged to the reservoir 28 by a pump 30 is returned to the accumulator 29 and the master cylinder 26. As a result, the braking pressure is decreased.
On the other hand, in the case where the command of increasing the braking pressure has been issued from the circuit 24, the solenoid driving circuit 25 de-energizes the solenoids SOL1 and SOL2, so that the pressure control valves V1 and V2 are returned to the positions shown in FIG. 1 and thus, the wheel cylinder 27 is communicated with the brake fluid pressure generator composed of the master cylinder 26 and the accumulator 29.
Meanwhile, when the command of maintaining the braking pressure has been given from the circuit 24, the solenoid driving circuit 25 energizes the solenoid SOL1 and de-energizes the solenoid SOL2. Thus, the pressure control valve V1 is displaced in the rightward direction in FIG. 1, while the pressure control valve V2 is held in the position shown in FIG. 1. Therefore, the wheel cylinder 27 is isolated from the brake fluid pressure generator, so that the pressure of the wheel cylinder 27 is contained in the circuit of the wheel cylinder 27 and thus, the braking pressure is maintained at a constant value.
A control means is provided in the antiskid device referred to above as disclosed, for example, in Japanese Patent Publication (examined) No. 30588/1984. In this prior art antiskid device, when the command of decreasing the braking pressure has been issued continuously for a first preset time period or more upon detection of the wheel state towards locking, the control means gives, for a second present time period after lapse of the first preset time period, the command of maintaining the braking pressure by suspending the command of decreasing the braking pressure. This action of the control means is performed for the following purpose. Namely, progress of decrease of the braking pressure is detected once in the course of decrease of the braking pressure and then, the braking pressure is further decreased if necessary such that a braking distance is shortened without decreasing the braking pressure excessively.
However, in the prior art antiskid device, if the braking pressure is maintained regardless of behaviors of the wheels upon lapse of the predetermined time period after start of decrease of the braking pressure as described above, such an undesirable phenomenon may take place that although the braking pressure is actually required to be decreased further, the wheels are locked exceedingly through maintenance of the braking pressure in too early a stage or conversely that in the case of the wheels which could have been recovered from locking even if the braking pressure had been maintained in an earlier stage, the braking pressure is decreased excessively since the braking pressure is maintained upon lapse of the predetermined time period.